CN118541814A - Graphene photodetector - Google Patents

Graphene photodetector Download PDF

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Publication number
CN118541814A
CN118541814A CN202280086611.5A CN202280086611A CN118541814A CN 118541814 A CN118541814 A CN 118541814A CN 202280086611 A CN202280086611 A CN 202280086611A CN 118541814 A CN118541814 A CN 118541814A
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graphene
metal electrode
channel
layer
electrode
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马尔科·罗马尼奥利
维托·索里亚内洛
西蒙娜·马尔科尼
阿莱西奥·皮拉斯图
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National Inter University Telecommunication Union
Kamglafik LLC
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National Inter University Telecommunication Union
Kamglafik LLC
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/08—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors
    • H01L31/10—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors characterised by potential barriers, e.g. phototransistors
    • H01L31/101—Devices sensitive to infrared, visible or ultraviolet radiation
    • H01L31/112—Devices sensitive to infrared, visible or ultraviolet radiation characterised by field-effect operation, e.g. junction field-effect phototransistor
    • H01L31/113—Devices sensitive to infrared, visible or ultraviolet radiation characterised by field-effect operation, e.g. junction field-effect phototransistor being of the conductor-insulator-semiconductor type, e.g. metal-insulator-semiconductor field-effect transistor
    • H01L31/1133—Devices sensitive to infrared, visible or ultraviolet radiation characterised by field-effect operation, e.g. junction field-effect phototransistor being of the conductor-insulator-semiconductor type, e.g. metal-insulator-semiconductor field-effect transistor the device being a conductor-insulator-semiconductor diode or a CCD device
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/02—Details
    • H01L31/0224—Electrodes
    • H01L31/022408—Electrodes for devices characterised by at least one potential jump barrier or surface barrier
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0352—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions
    • H01L31/035209—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions comprising a quantum structures

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  • Physics & Mathematics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • Electromagnetism (AREA)
  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
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Abstract

The graphene photodetector includes: a first graphene absorption layer (2), the first graphene absorption layer (2) being connected to a first metal electrode (3) and a second metal electrode (4), the first and second metal electrodes defining a channel (5) on the first graphene layer (2), the channel (5) operating as a plasma waveguide; a gate dielectric layer (6), the gate dielectric layer (6) being disposed between a first graphene layer (2) and a second graphene layer (7) for electrical gating, and the second graphene layer comprising a first gate electrode (8) and a second gate electrode (9), the first gate electrode (8) and the second gate electrode (9) being proximate to the first metal electrode (3) and the second metal electrode (4), respectively; -a photonic dielectric waveguide (10), the photonic dielectric waveguide (10) having a planar cladding layer (11), the planar cladding layer (11) being disposed below the gate dielectric layer (6); wherein the first gate electrode (8) and the second gate electrode (9) remain interposed between the gate dielectric layer (6) and the cladding layer (11); wherein the distance between the first metal electrode (3) and the second metal electrode (4) defines the width of the channel cross section, the distance between the first metal electrode (3) and the second metal electrode (4) being comprised between 100nm and 600 nm; and the distance between the first gate electrode (8) and the second gate electrode (9) is at least 60% of the distance between the first metal electrode (3) and the second metal electrode (4).

Description

Graphene photodetector
Technical Field
The present invention relates to a graphene photodetector, and in particular, to a graphene photodetector using an optical conversion mechanism based on a photo-thermal effect and a photovoltaic effect.
Background
Graphene photodetectors offer a number of advantages in a number of applications due to the nature of graphene, in particular for high speed data and telecommunications applications.
Graphene is a single atom thick carbon layer with a two-dimensional hexagonal structure hybridized with sp 2. The valence and conduction bands of this material intersect at six points in reciprocal space (reciprocal space), known as Dirac points.
Graphene has a low state density that varies linearly with electron state energy and disappears at the dirac point. This feature enables easy adjustment of the chemical potential (low gate voltage if compared to other materials like silicon is required to change the chemical potential) and all material properties associated therewith (e.g. conductivity, seebeck coefficient, optical absorption (optical absorptio), etc.) by means of field effect. This feature is called electrostatic doping.
The light absorption spectrum for graphene spans from UV to far IR, and when the material is properly encapsulated (e.g., in hBN), the mobility of charge carriers in graphene can exceed 100.000cm 2/Vs even at room temperature.
Due to the short relaxation time (in the order of picoseconds) and the small electronic heat capacity, the fast charge carrier dynamics of the optically excited charge carriers upon optical excitation enable photodetectors with a photo-electric bandwidth of more than 100 GHz.
In addition, graphene can be grown by Chemical Vapor Deposition (CVD) on a suitable substrate (e.g., copper) and transferred to virtually any photonic substrate.
The graphene photodetector based on the photo-thermal effect is mainly used for realizing direct optical power-to-voltage conversion, zero dark current and ultra-fast operation.
The photo-thermal effect (PTE) is based on the temperature rise of the electronic system after absorption of optical power. In PTE-based graphene photodetectors, the electromotive force is generated by the seebeck effect, which is caused by the spatial gradient of electron temperature in graphene, with spatially non-uniform seebeck coefficients.
To better understand this concept, assuming that the laser beam excites an active graphene layer with a spatially uniform chemical potential, and thus the seebeck coefficient (depending on the nature of the chemical potential) is constant along the channel, the hot carriers optically excited in the laser spot region (i.e., electrons and holes at a higher temperature relative to the lattice after irradiation) diffuse radially from the center to the sides of the excitation region. In this case, the hot electrons (or holes, depending on the sign of the seebeck coefficient) diffuse in opposite directions, producing zero net photocurrent. Conversely, if a step change in chemical potential (and thus in seebeck coefficient) is induced at the center of the excitation region, hot electrons or holes will diffuse in the same direction, producing a net photocurrent. Since the photo-response is directly generated by the thermoelectric effect, unlike other effects (e.g., photoconductive effect and photo-radiative thermal effect), the photo-thermoelectric effect does not require biasing the active graphene layer and thus can operate without dark current.
Both the photovoltaic effect and the photo-thermal effect require a non-uniform chemical potential, i.e. a pn junction, for different reasons. There is a significant difference between graphene pn homojunctions (where the junction is made of graphene only) and classical semiconductor-based diodes.
The first difference is that classical semiconductors require physical doping to achieve a pn junction. Differently, the easy tuning of the graphene chemical potential by field effect can be achieved by using a suitable gating structure (gating structure) to achieve an electrostatically induced pn-homojunction. Several gating configurations are reported in the literature, combining a top gate configuration (gate electrode is arranged on top of the active graphene layer) and a bottom gate configuration (gate electrode is arranged below the active graphene layer). For example only, reference is made below to a top split gate configuration and a bottom split gate configuration.
In a split gate configuration, two gate electrodes separated by a small gap (typically less than 300 nm) are used to induce in the active layer a spatial doping profile with opposite signs in both sides of the junction. Fig. 1a to 1d show two examples of top split gate configurations and bottom split gate configurations associated with waveguide-integrated graphene photodetectors.
In fig. 1a (bottom gate configuration, zero-bias detector operation) and fig. 1c (bottom gate configuration, no-bias operation), two portions of the doped silicon trench waveguide are used as gate electrodes under the active graphene channel. This solution avoids depositing a dielectric on top of the active layer, thus preventing a drop in its electrical characteristics. However, bottom gates limit the design to using doped silicon to implement the gate electrode. Other photon platforms (such as SiN) are not available. The use of chemical doping removes the advantage of graphene, as graphene does not require doping. Other possible bottom gate configurations for waveguide integrated photodetectors are not feasible because the conductive layer disposed between the waveguide and the active graphene layer will absorb a relatively large amount of optical power relative to the active channel. The optical power absorbed by the gate does not contribute to the photo-current, so that the responsiveness is reduced.
In the top split gate configuration of fig. 1b (top gate configuration, zero-bias operation) and fig. 1d (top gate configuration, no-bias operation), two graphene gate electrodes are arranged (place) on a thick gate dielectric (thickness greater than 100 nm) that is deposited on top of the active channel. Since the graphene gate is disposed at a large distance from the waveguide, the optical power absorbed in the gate layer accounts for about 10% of the total absorption. However, the mobility of the charge carriers is affected by the deposition process, as outlined in Giambra et al, optics express 27 (15), 20145-20155 (Giambra et al, optics express 27 (15), 20145 to 20155).
A second significant difference between graphene pn homojunctions and typical semiconductor pn junctions is that due to the semi-metallic nature of graphene, graphene pn homojunctions have no rectifying behavior. When a bias is applied to the graphene pn junction, a large current (even a current of mA-level, depending on the sample resistance and the applied bias) flows regardless of the polarity of the bias with respect to the p-side or n-side, i.e. no reverse bias condition is present to suppress the diode dark current. Thus, the only possibility to realize photodetectors operating with low or zero dark current is to use the photovoltaic effect or the photo-thermal effect, since these effects do not require a bias voltage.
Fig. 1a, 1b and 1c, 1d show the difference between the operation of two photodetectors: zero-bias operation and no-bias operation. In fig. 1a, 1b, the integrated photodetector operates in a zero-bias condition, i.e., the drain electrode (right metal electrode) is grounded by an inducer. In fig. 1c, 1d, the photodetector is directly connected to the electronics (amplifiers in the figures) for data readout without the application of an external bias. In both cases, only photocurrent is present (zero dark current operation). In unbiased operation, it is preferable to express the responsivity in terms of voltage responsivity (V/W) corresponding to the ratio of photovoltage to incident optical power.
Disclosure of Invention
The main object of the present invention is to provide a graphene photodetector that overcomes the limitations highlighted with reference to the known solutions.
This object, as well as other objects that will become more apparent hereinafter, is achieved by a graphene photodetector made according to the appended claims.
According to one aspect of the disclosed subject matter, the present invention relates to a graphene photodetector comprising:
A first graphene absorber layer connected to a first metal electrode at a first end of the first graphene layer and connected to a second electrode at a second end of the first graphene layer, the second end of the first graphene layer being opposite the first end, the first and second metal electrodes being referred to as a source and a drain, respectively,
The first metal electrode and the second metal electrode define a channel on the first graphene layer and also define a plasmonic waveguide,
A gate dielectric layer disposed between the first graphene layer and the second graphene layer,
The gate dielectric layer is disposed on an opposite side of the channel relative to the first graphene layer,
The second graphene layer is for electrical gating and the second graphene layer includes a first gate electrode and a second gate electrode, the first gate electrode and the second gate electrode being proximate to a first metal electrode and a second metal electrode, respectively,
The first gate electrode and the second gate electrode are centrally aligned with respect to the channel,
A photonic dielectric waveguide having a planar cladding layer disposed below the gate dielectric layer, wherein the first gate electrode and the second gate electrode remain disposed between the gate dielectric layer and the cladding layer,
The distance between the first metal and the second metal defines the width of the channel cross-section, the distance between the first metal and the second metal being comprised between 100nm and 600nm,
The distance between the first gate electrode and the second gate electrode is at least 60% of the distance between the first metal electrode and the second metal electrode.
In some embodiments, the width of the channel may be preferably comprised between 250nm and 450 nm.
In some embodiments, the thickness of the first and second metal electrodes defines a height of the channel cross section, the thickness of the first and second metal electrodes being comprised between 70nm and 200 nm.
In some embodiments, the first metal electrode and the second metal electrode preferably have a thickness of 100nm.
In some embodiments, the thickness of the gate dielectric layer is comprised between 10nm and 40 nm.
In some embodiments, the thickness of the dielectric layer is preferably 20nm.
In some embodiments, the first metal electrode and/or the second metal electrode is made of one or more of the following metals: gold, silver, aluminum, titanium nitride (TiN) or alloys thereof.
In some embodiments, the distance between the first metal electrode and the second metal electrode defining the width of the channel cross section is constant over the longitudinal extension of the channel.
In some embodiments, the constant width of the channel cross section is comprised between 250nm and 450 nm.
In some embodiments, the width of the channel varies periodically over the longitudinal extension of the channel, wherein the segments having the smallest width alternate with the segments having the largest width, and wherein the width varies gradually between the smallest and largest values along the longitudinal direction, and the width varies gradually between the largest and smallest values along the longitudinal direction.
In some embodiments, the minimum width is comprised between 100nm and 250nm and the maximum width is comprised between 450nm and 600 nm.
In some embodiments, the number of channel segments having the smallest width is comprised between 2 and 5.
In some embodiments, in the channel, three sections with minimum width are provided.
In some embodiments, between two sections adjacent to each other having a minimum width and a maximum width, opposite surfaces of the channel are angled at an angle between 4 ° and 23 ° degrees relative to a longitudinal extension direction of the channel.
In some embodiments, the optical mode of the dielectric waveguide must be quasi-transverse electric (quasi-TE).
In some embodiments, the channel may be implemented by using more than one graphene layer, preferably, the channel may be implemented by using two graphene layers. Preferably, two graphene layers are stacked on top of each other.
Drawings
Further features and advantages of the invention will become more apparent from the following detailed description of some of the preferred embodiments, which is illustrated by way of non-limiting example with reference to the accompanying drawings, in which:
figures 1a to 1d are schematic cross-sectional views showing various graphene photodetector embodiments according to the related art,
Figure 2 is a schematic cross-section of a graphene photodetector implemented according to the invention,
Figures 3 and 4 are schematic top views of various embodiments of photodetectors implemented according to the invention,
Figure 5 is a schematic top view of the particular enlarged scale shown in figure 4,
Fig. 6 is a graph showing the ratio between the power absorbed by the active graphene layer and the power absorbed by the illustrated graphene gate electrode versus gate dielectric thickness (versus) in the photodetector of the present invention,
Fig. 7 is a schematic top view, on an enlarged scale, showing the area of the gap between the metal electrodes provided in the active graphene channels of the photodetectors of the present invention,
Figure 8 is a graph showing the density of optical power absorbed at the gold/graphene interface for a selected gap width in the photodetector of the invention,
Figure 9 is a graph showing the voltage responsivity as a function of gap width in the photodetector of the invention,
Figure 10 is a graph showing the power absorbed by the metal and the power absorbed in the active graphene channel of the photodetector as a function of the corresponding function of the thickness of the metal electrode,
Figure 11 is a graph showing the light absorption in the active graphene channel of a photodetector versus the distance between the dielectric layer and the dielectric waveguide,
Fig. 12 is a graph showing the light absorption in the active graphene channel of a photodetector as a function of the thickness of the dielectric layer.
Detailed Description
Referring first to fig. 2, a graphene photodetector implemented in accordance with an embodiment of the present invention is indicated generally at 1. In fig. 2, a schematic view of a cross section of the photodetector 1 is shown.
The photodetector 1 comprises a first graphene absorption layer 2 (having a planar configuration depicted with a dashed line), which first graphene absorption layer 2 is connected to a first metal electrode 3 at a first end 2a of the first graphene layer 2, and which first graphene absorption layer 2 is connected to a second metal electrode 4 at a second end 2b of the first graphene layer 2, which second end 2b of the first graphene layer 2 is opposite to the first end 2 a. The first metal electrode 3 and the second metal electrode 4 are called a source and a drain, respectively.
The contact between the graphene layer 2 and each of the metal electrodes 3, 4 ensures a suitable electrical connection to conduct and detect photocurrent generated in the photodetector.
The first metal electrode 3 and/or the second metal electrode 4 are preferably made of one or more of the following metals: gold, silver, aluminum, titanium nitride (TiN), or alloys thereof.
The first metal electrode 3 and the second metal electrode 4 also define a channel 5 on the first graphene layer 2, said channel 5 operating as a plasmonic waveguide, as clearly disclosed below.
The first metal electrode 3 and the second metal electrode 4 are spaced apart and the distance between the first metal electrode and the second metal electrode (denoted d 1) defines the width of the channel cross-section.
The thickness of the first and second metal electrodes (denoted t m) defines the height of the channel cross-section, and preferably the thickness of the first and second metal electrodes is comprised between 70nm and 200nm, and more preferably the thickness of the first and second metal electrodes is 100nm.
Preferably, the distance d 1 between the first metal electrode 3 and the second metal electrode 4 is comprised between 100nm and 600nm, and more preferably, the distance d 1 between the first metal electrode 3 and the second metal electrode 4 is comprised between 250nm and 450 nm.
The photodetector 1 further comprises a gate dielectric layer 6, which gate dielectric layer 6 is placed between the first graphene layer 2 and a second graphene layer 7 (also depicted with dashed lines), this configuration realizing a capacitor, wherein the dielectric layer 6 is arranged on the opposite side of the channel 5 with respect to the first graphene layer 2. Preferably, the dielectric layer 6 is made of SiN or Al 2O3.
Preferably, the first graphene layer 2 and the second graphene layer 7 are planar and the first graphene layer 2 and the second graphene layer 7 are parallel to each other, the distance between them (denoted as t diel) being defined by the thickness of the dielectric layer 6.
The second graphene layer 7 is used for electrical gating (ELECTRICAL GATING), and the second graphene layer 7 comprises a first gate electrode and a second gate electrode denoted by 8,9, which are positioned proximate (proximate to) the first metal electrode 3 and the second metal electrode 4, respectively, in an at least partially overlapping manner with the first graphene layer 2.
Preferably, the first gate electrode 8 and the second gate electrode 9 are spaced apart by a distance d 2, and the first gate electrode 8 and the second gate electrode 9 have a centrally aligned configuration with respect to the channel 5, as clearly shown in fig. 2. The centrally aligned configuration means that the gate electrodes 8, 9 are arranged in a mirror-symmetrical manner with respect to a hypothetical midline symmetry plane of the cross-sectional channel 5, which is identified by the axis indicated in fig. 2 as Z.
Preferably, the distance d 2 between the gate electrodes 8, 9 is at least 60% of the distance between the first metal electrode 3 and the second metal electrode 4, and more preferably, the distance d 2 is comprised between 100nm and 300 nm. Within this range, the value of d 2 is more preferably 150nm.
As further disclosed below, the metal electrodes 3, 4 defining the active channel 5 on top of the first graphene layer 2 are provided for collecting photocurrent and confining light at the metal-graphene interface. Control of the electrostatic doping in the active channel by changing the graphene chemical potential (applying an external voltage to the gate electrode) is achieved by using the so-called bottom split gate geometry obtained by the gate electrodes 8,9 of the second graphene layer 4.
The photodetector 1 further comprises a photonic dielectric waveguide 10, the photonic dielectric waveguide 10 having a planar cladding layer 11, the planar cladding layer 11 being arranged below the dielectric layer 6, wherein the first gate electrode 8 and the second gate electrode 9 remain interposed between the dielectric layer 6 and the cladding layer 11. The waveguide 10 comprises a core 12, preferably a silicon core, embedded in a cladding 11, preferably the cladding 11 is a SiO 2 cladding.
The waveguide 10 is positioned centrally aligned (centrally) with respect to the active graphene channel 5, preferably the waveguide 10 is constructed with a rectangular cross-section. The dielectric spacer thickness between the waveguide 10 and the graphene gate electrode is denoted by t clad. Preferably, the waveguide 10 may have a rectangular cross section of 220nm×480 nm.
Referring to the top view of fig. 3, the active channel 5 and the waveguide 10 extend along a dominant longitudinal direction (PREVAILING LONGITUDINAL DIRECTION) in the figure, identified by the Y-axis. X represents a direction perpendicular to the Y axis and pointing parallel to the first graphene layer 2. The distance d 1 is measured in the lateral direction X, the distance d 1 defining the gap between the metal electrodes 3,4 in the active graphene channel 5.
According to one embodiment of the invention, the distance d 1 between the first metal electrode 3 and the second metal electrode 4 defining the width of the active graphene channel 5 is constant along the longitudinal extension Y of the channel, as shown in the top view of fig. 3. This configuration is obtained by having facing edges of the respective metal electrodes 3, 4 parallel to each other and spaced apart by a gap distance d 1 for the dominant longitudinal extension.
According to another embodiment of the invention, as shown in the top view of fig. 4, the distance d 1 defining the width of the channel 5 may be periodically varying over the longitudinal extension of the channel, wherein channel sections having a minimum width denoted d 1 ' alternate with sections having a maximum width denoted d 1 ", and wherein the width gradually varies between a minimum value d 1 ' and a maximum value d 1" along the longitudinal direction Y, and the width gradually varies between a maximum value d 1 "minimum and a minimum value d 1 ' along the longitudinal direction Y.
Preferably, the minimum width d 1' is comprised between 100nm and 300nm and the maximum width d 1 "is comprised between 450nm and 600 nm.
Preferably, the number of channel sections having a minimum width d 1 'may be comprised between 2 and 5, and more preferably three channel sections having a minimum width d 1' may be provided over the longitudinal extension of the channel.
In fig. 5, the tapered configuration of the channel section in fig. 4 is shown on an enlarged scale. In the tapered configuration, the opposing surfaces of the channel are angled at an angle α relative to the longitudinal extension direction of the channel.
In order to efficiently convert the dielectric waveguide mode into a plasma mode of the detector structure, a small taper angle α (see fig. 5) is required. However, too small a taper angle results in a longer taper section (in the propagation direction y). This would be detrimental as the loss of metal would increase as the resulting responsiveness decreases. The preferred angle α is defined in the range between 4 ° and 23 °.
The distance between the gate electrode and the dielectric waveguide (t clad, see fig. 2) must be small enough to ensure good coupling between the dielectric waveguide and the detector stack. However, the gate electrode is electrically isolated. Thus t clad +.0.
The thickness t diel of the gate dielectric layer is selected to be small enough to maximize light absorption in the active graphene channel. Preferably, however, the thickness t diel is selected to be at least 20nm to prevent current leakage between the active channel and the gate electrode.
Graphene-based photodetectors of the claimed subject matter are presented for utilizing the light conversion mechanisms (photovoltaic and photo-thermoelectric effects) that occur at the metal/graphene interface. Photocurrent can be generated using photovoltaic and photo-thermal mechanisms at the metal/graphene interface. Unlike the related art devices using graphene homojunctions described with reference to fig. 1a to 1d, the photovoltaic effect is expected to make a related contribution in addition to the photo-thermal effect. The basic idea behind the claimed photodetector is to use plasmonic waveguides to confine the optical field to the edges of the metal electrodes (source and drain) used to collect the photocurrent. The photodetector structure is designed to be integrated on top of a photonic dielectric waveguide 10 with a planar cladding layer 11 and is made of a stack of two graphene layers 2, 7 separated by a dielectric layer 6. The metal electrodes 3,4 on top of the first graphene layer 2 (active channel 5) serve to collect photocurrent and confine light at the metal/graphene interface. The control of the doping in the active channel 5 (first graphene layer 2) is achieved by using the bottom split gate geometry obtained by the second graphene layer 7.
The geometry of the photodetector is shown in fig. 2-4, where the device stack is integrated on top of a photonic waveguide 10 with a core 12 and a planar cladding 11. The source electrode 3 and the drain electrode 4 serve both as electrodes to collect photocurrent and as plasmon waveguides to confine light at the metal/graphene interface.
In order to excite the plasma modes, the optical modes of the dielectric waveguide must be quasi-transverse electric (quasi-TE).
The light from the dielectric waveguide 10 couples into the plasmonic mode of the metal-insulator-metal (MIM) waveguide on top of the active graphene channel 5. Most of the optical power is absorbed at the graphene/metal interface at the edge of the metal contact. Referring to fig. 3, the MIM has a width d 1 of 300nm before the metal absorption dominates the graphene absorption.
In fig. 4, the distance between metals varies periodically, with regions of MIM having larger widths (d 1 greater than 300 nm) alternating with regions of smaller widths (d 1 less than 300 nm).
As described above, referring to the related art solution, the graphene layer interposed between the dielectric waveguide and the active graphene layer is detrimental because it absorbs a large amount of optical power, thereby reducing the responsivity of the photodetector. In the proposed invention this problem is greatly alleviated. Indeed, the use of plasmonic waveguides enhances the electric field in the active graphene layer. Furthermore, the light absorption of graphene is linearly proportional to the number of layers. By using two graphene layers, the active channel has a greater absorptivity with respect to the gate.
In the graph of fig. 6, the ratio between the power absorbed by the active graphene layer and the power absorbed by the graphene gate electrode is shown as a function of gate dielectric thickness (t diel). In this case, t clad is always 20nm. The ratio of the power absorbed by the active layer to the power absorbed by the graphene gate electrode spans from slightly above 400% for a 20nm thick layer to slightly below 200% for an 80nm thick gate dielectric. The graph has a monotonically decreasing trend, indicating that the graphene gate absorbs a significant portion of the optical power as the thickness of the gate dielectric (t diel) increases.
As for graphene active channels, the major drawbacks of small gaps between metal electrodes are large absorption in the metal and non-trivial (non-triple) control of the graphene chemical potential between two metal electrodes.
For a small gap, as observed by the applicant for a gap of 20nm, the chemical potential in the gap is almost constant and does not change from left to right contact. Since the gap region of the channel is the region that absorbs the largest part of the optical power, it is not possible to maximize the PTE and PV light response if the chemical potential in the gap cannot be controlled. Therefore, voltage responsiveness is poor. However, embodiments with tapered cross sections have the advantage of increasing the amount of optical power absorbed in the active graphene channel due to the field enhancement obtained in the gap. Two embodiments of photodetectors may be compared: 1-implementation of a photodetector with a constant width of 300 nm; and 2-photodetectors having a tapered cross section, for example, photodetectors having a minimum width d 1' equal to 250nm and a maximum width d 1 "equal to 600 nm. In a constant width photodetector implementation, the absorbed optical power is small compared to the case of photodetectors having a periodically tapered width. Furthermore, due to the minimum gap width (> 100 nm), in a periodically tapered implementation, the optical power is not only limited to in the gap, but the relevant part of the absorption also occurs in the section of the taper with the larger width. Fig. 8 shows the optical power absorbed at the metal graphene interface as a function of the Y-coordinate, which fig. 8 highlights. In this coordinate system, y=0 corresponds to the middle of the structure shown in fig. 7. For a taper with a minimum gap width of 20nm, the optical power is almost completely absorbed in the gap region. For devices with minimum gap widths of 250nm and 70nm, power is absorbed more uniformly along Y.
The absorption of optical power in the region of width greater than 100nm allows for more precise control of chemical potential. This allows for a better optimization of the PTE and PV effects and thus the responsivity of the detector can be optimized.
For this reason, solutions with tapered cross sections and relatively large gaps (> 100 nm) represent the best designs and thus define the best ranges for d 1' and d 1 "(see fig. 4).
In the graph of fig. 9, the analog voltage responsivity is shown as a function of gap width.
Fig. 10 is a graph showing the power absorbed by the metal and the power absorbed in the active channel as a function of metal electrode thickness t m, where t clad is 20nm and t diel is also 20nm. It can be observed that as the thickness of the metal increases, the power absorption in the metal decreases.
Fig. 11 is a graph showing the relationship of optical absorption in an active graphene channel with distance t clad, where t diel is 20nm and t m is 70nm. The distance t clad must be chosen as thin as possible to maximize the optical absorption in the active graphene channel. It can be observed that for t clad =50 nm, the absorbed power in the active channel is reduced by 54% relative to t clad =20 nm.
Fig. 12 is a graph showing the relationship of the optical absorption in the active channel to the thickness t diel of the dielectric layer, where t clad is 20nm and t m is 70nm. It can be observed that for t diel =50 nm, the absorbed power in the active channel is reduced by 60% with respect to t diel =20 nm.

Claims (16)

1. A graphene photodetector, the graphene photodetector comprising:
A first graphene absorption layer (2), the first graphene absorption layer (2) being connected to a first metal electrode (3) at a first end (2 a) of the first graphene layer (2), and the first graphene absorption layer (2) being connected to a second electrode (4) at a second end (2 b) of the first graphene layer, the second end (2 b) of the first graphene layer being opposite to the first end (2 a), the first metal electrode (3) and the second metal electrode (4) being referred to as source and drain respectively,
The first metal electrode (3) and the second metal electrode (4) define a channel (5) on the first graphene layer (2), the channel (5) operating as a plasmonic waveguide,
A gate dielectric layer (6), the gate dielectric layer (6) being disposed between the first graphene layer (2) and a second graphene layer (7),
The gate dielectric layer (6) is arranged on the opposite side of the channel (5) with respect to the first graphene layer (2),
The second graphene layer (7) is used for electrical gating and the second graphene layer (7) comprises a first gate electrode (8) and a second gate electrode (9), the first gate electrode (8) and the second gate electrode (9) being close to the first metal electrode (3) and the second metal electrode (4), respectively,
The first gate electrode (8) and the second gate electrode (9) are centrally aligned with respect to the channel (5),
-A photonic dielectric waveguide (10), the photonic dielectric waveguide (10) having a planar cladding layer (11), the planar cladding layer (11) being arranged below the gate dielectric layer (6), wherein the first gate electrode (8) and the second gate electrode (9) remain interposed between the gate dielectric layer (6) and the cladding layer (11),
The distance between the first metal electrode (3) and the second metal electrode (4) defines the width of the channel cross section, the distance between the first metal electrode (3) and the second metal electrode (4) is comprised between 100nm and 600nm,
The distance between the first gate electrode (8) and the second gate electrode (9) is at least 60% of the distance between the first metal electrode (3) and the second metal electrode (4).
2. The graphene photodetector according to claim 1, wherein preferably the width of the channel (5) is further comprised between 250nm and 450 nm.
3. The graphene photodetector according to claim 1 or 2, wherein the thickness of the first metal electrode (3) and the second metal electrode (4) defines the height of the channel section, the thickness of the first metal electrode (3) and the second metal electrode (4) being comprised between 70nm and 200 nm.
4. A graphene photodetector according to claim 3, wherein the thickness of the first metal electrode (3) and the second metal electrode (4) is preferably 100nm.
5. The graphene photodetector according to one or more of the preceding claims, wherein the thickness of said gate dielectric layer (6) is comprised between 10nm and 40 nm.
6. The graphene photodetector according to one or more of the preceding claims, wherein preferably the thickness of the dielectric layer (6) is 20nm.
7. The graphene photodetector according to one or more of the preceding claims, wherein the first metal electrode (3) and/or the second metal electrode (4) are made of one or more of the following metals: gold, silver, aluminum, titanium nitride (TiN) or alloys thereof.
8. The graphene photodetector according to one or more of the preceding claims, wherein the distance (d 1) between the first metal electrode (3) and the second metal electrode (4), defining the width of the channel section, is constant over the longitudinal extension (Y) of the channel (5).
9. The graphene photodetector of claim 8, wherein the constant width of the channel cross section is comprised between 250nm and 450 nm.
10. The graphene photodetector according to one or more of claims 1 to 7, wherein the width of the channel (5) varies periodically over the longitudinal extension (Y) of the channel, wherein the sections with minimum width (d 1') alternate with the sections with maximum width (d 1 "), and wherein the width varies gradually along the longitudinal direction between a minimum value and a maximum value, and the width varies gradually along the longitudinal direction between the maximum value and the minimum value.
11. The graphene photodetector according to claim 10, wherein the minimum width (d 1') is comprised between 100nm and 250nm and the maximum width (d 1 ") is comprised between 450nm and 600 nm.
12. The graphene photodetector according to claim 10 or 11, wherein the number of channel sections having the minimum width (d 1') is comprised between 2 and 5.
13. The graphene photodetector according to claim 12, wherein in the channel three sections with the minimum width (d 1') are provided.
14. The graphene photodetector according to claim 10, wherein between two sections adjacent to each other having a minimum width (d 1') and a maximum width (d 1 "), opposite surfaces of the channel (5) are angled at an angle (a) between 4 ° and 23 ° degrees with respect to the longitudinal extension direction of the channel (5).
15. The graphene photodetector according to one or more of the preceding claims, wherein the optical mode of the dielectric waveguide must be quasi transverse electric (quasi TE).
16. A graphene photodetector according to one or more of the preceding claims, wherein said channel (5) can be realized by using more than one graphene layer, preferably said channel (5) can be realized by using two graphene layers.
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